ortf-3d: ambience recording for 3d audio & vrsolution-but it does not, in itself, produce...

SCHOEPS GmbH · Karlsruhe, Germany · www.schoeps.de · www.facebook.com/SchoepsMics 1

Contents

One recording method for all 3D formats? 2

Is first-order Ambisonics adequate for 3D? 2

Criteria for stereophonic arrays 3

The ORTF-3D recording method 4

Translating theory into practice 5

Conversion for Dolby Atmos and Auro3D 5

Conversion for VR 6

Recording engineers who work with 3D sound face a diffi-cult task when choosing a suitable recording technique. The number of channels is greater than with playback systems that operate only in the horizontal plane, so the complexity increases as well.

When a customer demands 3D audio rather than conven-tional 5.1 surround it may be tempting to apply solutions that are overly simple. But when a 3D recording has been made well, using a suitable recording technique, the advantages are impressively audible.

Development and application of a stereophonic multichannel recording technique for 3D Audio & VR

Helmut Wittek, Schoeps Mikrofone, 2016

ORTF-3D: Ambience recording for 3D Audio & VR

Fig. 1: ORTF-3D arrangement, in a windscreen with the cover removed



One recording method for all 3D formats?There are various 3D audio playback systems, so the recording techniques

that work best for each of them will naturally be different. For soundfield

synthesis systems, multi-channel microphone arrays can be a solution,

while for 3D stereo, stereophonic miking techniques are the norm.

For binaural reproduction in the simplest case, a dummy head can be

used.

But all these systems share one requirement when recording complex,

sustained sound sources such as ambient sound: Stereophonic techniques

must be used, because they alone offer both highquality sound and high

channel efficiency. It is impossible or inefficient to reproduce in high qual-

ity the sound of a large chorus, for example, or the complex, ambient

sound of a city street, by compiling single point sources recorded with

separate microphones.

In the same way, multi-channel microphone arrays for soundfield syn-

thesis, such as higher-order Ambisonics („HOA“) or wavefield synthe-

sis, fall short in practice because their channel efficiency and sonic quality

are too low. If on the other hand the number of channels is reduced, e.g.

with first-order Ambisonics, the spatial quality becomes burdened with

compromise.

For binaural playback, the dummy head technique is clearly the simplest

solution-but it does not, in itself, produce results compatible with virtual

reality glasses, in which the binaural signals must respond to user‘s head

motions. This is possible only through the “binauralization” of a stereo-

phonic array—a technique that is already well established in audio for

games.

Is first-order Ambisonics adequate for 3D?There is a common assumption that Ambisonics would be the method of

choice for 3D and VR. The professional recording engineer would do well

to examine the situation more closely.

Ambisonics, which has existed for a long time by now, is a technology for

representing and reproducing the sound field at a given point. But just as

with wavefield synthesis, it functions only at a certain spatial resolution or

“order”. For this reason, we generally distinguish today between “first-or-

der” Ambisonics and “higher-order” Ambisonics (“HOA”).

3D AUDIO

The new approaches included in „3D Audio“ reproduce sound from

all spatial directions. This includes the Dolby Atmos and Auro3D

stereophonic systems; binaural / virtual reality („VR“) systems; and

sound-field synthesis approaches such as Ambisonics and wave-

field synthesis systems.

3D Audio can give distinctly better spatial perceptions than 5.1. Not

only is the elevation of sound sources reproduced, but noticeable

improvements can also be achieved with regard to envelopment,

naturalness, and accuracy of tone color. The listening area can also

be greater; listeners can move more freely within the playback room

without hearng the image collapse into the nearest loudspeaker.

First-order Ambisonics cannot achieve error-free audio reproduction, since

the mathematics on which it is based are valid only for a listening space

the size of a tennis ball. Thus the laws of stereophony apply here—a

microphone for first-order Ambisonics is nothing other than a coinci-

dent microphone with the well-known advantages (simplicity; small num-

ber of recording channels; flexibility) and disadvantages (very wide, impre-

cise phantom sound sources; deficient spatial quality) of that approach in

general.

Creation of a Ambisonics studio microphone with high spatial resolution

is an unsolved problem so far. Existing Ambisonics studio microphones are

all first-order, so their resolution is just adequate for 5.1 surround but too

low for 3D audio. This becomes evident in their low inter-channel signal

separation as well as the insufficient quality of their reproduced spatiality.

The original first-order Ambisonics microphone was the Soundfield micro-

phone, built the same way as for example the Tetramic or the new Senn-

heiser VR microphone. The Schoeps “Double M/S System” works in similar

fashion, but without the height channel.

Ambisonics is very well suited as a storage format for all kinds of spatial

signals, but again, only if the order is high enough. A storage format with

only four channels (first-order Ambisonics calls them W, X, Y, Z) makes

a soup out of any 3D recording, since the mixdown to four channels

destroys the signal separation of the 3D setup.

Ambisonics offers a simple, flexible storage and recording format for

interactive 360° videos, e.g. on YouTube. In order to rotate the per-

spective, only the values of the Ambisonics variables need be adjusted.

Together with the previously mentioned small first-order Ambison-

ics microphones, 360° videos are very easily made using small, portable

cameras.



As a result, most of the advantages of first-order Ambisonics do not come

into play in VR. On the contrary, its disadvantages (poor spatial quality,

crosstalk among virtual loudspeaker signals) only become more prominent.

If practical conditions allow for a slightly larger microphone arrangement,

an ORTF-3D setup would be optimal instead as an ambience microphone

for VR.

Criteria for stereophonic arraysStereophonic arrays are thus the approach of choice for all 3D for-

mats. The requirements for 3D are the same as in two- and five-channel

stereophony:

• Signal separation among all channels in order to avoid comb

filtering: No one signal should be present at significant levels

in more than two channels.

• Level and/or arrival time differences between adjacent

channels to achieve the desired imaging characteristics.

• Decorrelation of diffuse-field sound for optimal envelopment

and sound quality.

2-channel stereophony

These demands are still easy to fulfill in two-channel stereophony; a suit-

able arrangement of two microphones and two independent channels can

provide the desired imaging curve. Tools such as the Imaging Assistant

application (available as an iOS app or on the Web at www.ima.schoeps.

de) have been developed for this purpose. They take into account not only

the creation of phantom image sources, but also the ever-important chan-

nel decorrelation.

Fig. 2: Two-channel ORTF system in a suspension designed for use within

a windscreen; two cardioids,17 cm, 110º.

A classic, positive example is the ORTF technique, which has a 100º

recording angle and delivers a stereo signal with good channel

decorrelation.

5-channel stereophony

The above requirements are distinctly more difficult to meet with five

channels, and there are numerous geometries that fail to meet them, e.g.

a microphone that looks like an egg the size of a rugby ball, with five

omni capsules that can deliver only a mono signal at low frequencies.

Five independent channels simply cannot be obtained with any coinci-

dent arrangement of first-order microphones. A coincident arrangement

such as first-order Ambisonics is thus a compromise for 5.1, though highly

workable because of its advantages in compactness and post-production

flexibility.

One optimal solution for ambient recordings in multi-channel stereophony

is the “ORTF surround” system, in which four supercardioids are arranged

in a rectangle with 10 x 20 cm side lengths. Here the distances between

microphones help with decorrelation, and thereby lend the sonic impres-

sion its spatial openness. The microphone signals are routed discretely to

the L, R, LS and RS channels. The signal separation in terms of level is ca.

10 dB; thus the sonic image during playback is stable even in off-axis lis-

tening positions

Fig. 3: Four-channel “ORTF Surround” system; four supercardioids, 10 / 20

cm spacing, 80º / 100º angles.



8 or more channels

With eight or nine channels, the arrangement of the microphones

becomes very difficult if the above-mentioned requirements are to be met.

The simplest method for maintaining signal separation is to set up eight

or nine microphones far apart from one another. Thus a large nine-chan-

nel “Decca Tree” arrangement is very well suited for certain applications,

although it has severe disadvantages that limit its practical usability. For

one, the sheer size of the arrangement is greater than 2 meters in width

and height. And the signal separation in terms of level difference is nearly

zero; every signal is more or less available in all loudspeakers. Thus this

array can represent a beautiful, diffuse spaciousness, but stable directional

reproduction isn’t achieved beyond the “sweet spot.” This can be helped

by adding spot microphones.

The ORTF-3D recording method An optimal ambience arrangement for eight channels is offered by the new

“ORTF-3D” system (developed by Helmut Wittek and Günter Theile). It is

more or less a doubling of the “ORTF Surround” system onto two planes, i.e.

there are four supercardioids on each level (upper and lower), forming rect-

angles with 10 and 20 cm side lengths. The two “ORTF Surround” arrange-

ments are placed directly on top of one another, see Fig. 6.

The microphones are furthermore tilted upward or downward in order to

create signal separation in the vertical plane, see Fig. 5. Thus an 8-channel

arrangement is formed, with imaging in the horizontal plane that somewhat

corresponds to the “ORTF Surround” system. The microphone signals are

discretely routed to four channels for the lower level (L, R, LS, RS), and four

for the upper level (Lh, Rh, LSh and RSh). In VR applications, virtual loud-

speaker positions forming an equal-sided cube are binauralized.

Fig. 4: A prototype of the ORTF-3D system at the ICSA conference in

2015. Eight supercardioids, horizontal distance 20 cm, vertical distance 0,

angle 90º.

Imaging in the vertical dimension is produced by angling the microphones

into 90-degree X/Y pairs of supercardioids. Such a two-channel coincident

arrangement is possible due to the high directivity of the supercardioids,

and the imaging quality and diffuse-field decorrelation are both good.

Fig. 5: Orientation of the capsules: one vertical X/Y microphone pair for

each vertical pair of loudspeakers

This results in an eight-channel array with high signal separation, opti-

mal diffuse-field correlation, and high stability within the listening area.

All requirements are optimally fulfilled, yet the array is no larger than the

compact ORTF Surround system—a decisive practical advantage. Numer-

ous test recordings have shown that the ORTF-3D approach produces very

beautiful, spatially open and stable 3D recordings.



Translating theory into practiceFor the SCHOEPS ORTF-3D Outdoor Set, eight compact supercardioid

CCM studio microphones are used. All microphones, as well as the wind-

screen itself, are elastically suspended in order to decouple vibrations.

Each vertical X/Y pair is composed of one front-addressed CCM 41 and

one radially-addressed CCM 41 V. This enables a space-saving parallel

arrangement of the microphone housings.

The windscreen and suspension have been developed by Schoeps

together with the company Cinela. As with the “ORTF Surround” wind-

screen, elastic suspensions are also available for the ORTF-3D wind-

screen; fur, optional rain protection, multicore cables with breakout cables

and integrated heating are standard. The windscreen is designed to be

mounted by hanging. Thus long-lasting outdoor installations, e.g. from

the roof of a stadium, are possible.

Fig. 6: ORTF-3D windscreen with the cover removed; view from below

This microphone arrangement, which was initially introduced as a pro-

totype at the end of 2015, has already been sold or rented in consider-

able numbers to customers in the sports and VR sectors. Tests have been

made with great success during the past two years, including several well-

known sporting events

Conversion for Dolby Atmos and Auro3DThe eight channels of the ORTF-3D are L, R, LS, RS for the lower level, and

Lh, Rh, LSh and RSh for the upper level. They are routed to eight discrete

playback channels without matrixing.

The center channel remains unoccupied. A center channel is seldom

desired in ambience recording; it would distort the energy balance

between front and rear, and require significantly greater distances among

microphones in order to maintain the necessary signal separation. If a cen-

ter signal should be necessary for a specific reason, e.g. to cover the shut-

off of a reporter’s microphone, a simple downmix of the L and R signals at

low level is sufficient.

In Auro3D the loudspeaker channels L, R, LS, RS, HL, HR, HLS and HRS are

fed.

With Dolby, the integration in the Atmos production environment is

equally simple; the channels L, R, LS, RS are simply laid down in the cor-

responding channels of the surround level, the so-called “Atmos bed,”

whereas the four upper channels are placed as static objects in the four

upper corners of the Cartesian space in the Atmos panning tool. These

are then rendered in playback through the corresponding front or rear

loudspeakers.

The below screen capture from ProTools, with the four Atmos

panners as well as the monitoring application, illustrates this.

Fig. 7: Routing of the eight channels from the ORTF-3D in Dolby Atmos (ProTools plugin)



Conversion for VRIn a virtual reality (“VR”) environment, 3D video and binaural sound are

reproduced via VR glasses with headphones. Head position and rotation

are processed in real time. 360° videos can also contain binaural sound,

but only head rotation is processed, not the head position.

Fig. 8: VR glasses (Samsung)

If binaural sound is to respond to head tracking, a dummy head cannot

be used as the recording method since it allows only for one head angle.

Instead, the following sound components are gathered separately and

assembled:

• “Audio object” with dry sound

• Binaural filters: “HRTF” (with room: “BRIR”)

Usually the audio object e.g. a character in a VR video game, is a single

source with a certain distance and 3D direction. It consists of dry sound,

which is then processed via binaural and room filters (=”binauralized”)

depending on its 3D direction. This direction is determined by the posi-

tion of the audio object and the position and head rotation of the listener

within the VR scene.

The acoustical background signal of a scene, or “ambience/atmo”, is a

very special kind of audio source. It cannot be recorded dry, nor can it be

mapped to a single point source. In principle it could be produced by the

superposition of numerous audio sources in space, but often this would

either be inefficient (e.g. trees in a forest) or impossible (live ambience

from a venue).

Thus a group of several audio objects forming an array of virtual loud-

speakers is used to reproduce a stereophonic recording of the ambience.

These group of loudspeakers can be chosen from a 3D preset, for example

the Dolby setup 5.1.4, or the Auro3D setup 9.1, in each case without a

center loudspeaker. If no preset is available, one can define an equal-sided

cube around the listener.

These audio objects are “diegetic”, i.e. they do - exactly as their visual

counterparts - not move in response to head rotation. This does imply that

their incidence angle in relation to the head changes with head rotations

and thus the HRTFs change. The eight signals of the ORTF-3D microphone

are utilized in this way to build up an optimal 3D live ambience in the VR

environment.

The use of a first-order Ambisonic microphone for this purpose cannot be

recommended as described above. Being a small, coincident setup, its out-

put lacks sufficient separation among channels, thus reducing the quality

of its spatiality and 3D stereophonic imaging.

Fig. 9: Virtual 8.0 loudspeaker setup to reproduce live recorded ambience

within a binaural environment

For more information:

www.hauptmikrofon.de/stereo-3d/3d-audio/ortf-3d

ortf-3d: ambience recording for 3d audio & vrsolution-but it does not, in itself, produce...

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